The present work builds upon a few key results within the field of wireless networks. The first is the observation that vehicular networks suffer from the gray-zone phenomenon, a problem that existing vehicular protocols do not address. Second, the idea that radio diversity can be used to opportunistically tackle unreliable channels has been proposed in the context of WLANs and mesh networks, but not in the vehicular network context.
The existence of a large gray-zone of partial connectivity in VANETs was first pointed out by Bai et al[3]. In their experiments they found the probability of having an intermediate packet delivery rate between 20 and 80% to be 50%.
Kaul et al[17] studied the effect of multi-radio diversity using antennas placed in different parts of a vehicle. In their experiments they reported a 10-15% packet error rate reduction by adding a second radio. Given that the antennas were placed very close to each other, this can be seen as a lower bound on the benefits of radio diversity on vehicular environments.
Most VANET routing protocols do not use diversity. Instead, they focus on a neighbor-based strategy of choosing a single relay per hop, differing mostly in the metric used for relay selection. GPSR [10], A-STAR [4] and Gytar [5] choose the neighbor closest to the destination, a risky choice given the gray-zone phenomenon present in VANETs. ACAR [12] uses a modified Expected Transmission Count (ETX) metric [18] that tries to minimize the end-to-end error probability. This is a good improvement but still has a single point of failure.
BLR [19] and CBF [20] are two VANET protocols where forwarding decisions are made on the receiver side. However, they are susceptible to replication and unable to limit the number of forwarders to reduce contention in high-density environments. DOT [21], establishes a prioritization, but does not limit the number of forwarders.
Diversity has previously been used in other contexts to recover from losses. Multi-Radio Diversity (MDR) [8] is a low-level scheme for WLANs where corrupt frames received at different APs are combined in a central node to try and extract a correct frame from the multiple corrupt copies. This scheme requires a shared channel to a central node, rendering it unsuitable for vehicular use.
Opportunistic routing has also been explored in the context of mesh networks, with the most prominent protocols being ExOR [6] and MORE [22]. Both leverage diversity by using multiple relays and both assume network-wide knowledge of channel quality between every pair of neighbors, which is reasonable for mesh networks but does not hold in VANETs.
PRO [7] is a distributed opportunistic scheme for infrastructure WLANs. In PRO, when a transmission fails, relays that have a good RSSI towards the destination opportunistically retransmit the packet on behalf of the source, increasing reliability. PRO requires nodes to learn the RSSI between all sources and destinations. While this is feasible in WLANs, all nodes in VANETs can be senders and receivers, plus channels are very dynamic. DAZL instead ranks relays based on distance rather than RSSI.
The idea of avoiding MAC layer contention by reducing the number of candidate transmitters first appeared as an answer to the broadcast storm problem [23]. Some schemes, such as SAPF [24] and P-persistence [25] use a simple probabilistic rule to control the number of forwarders, without prioritization. Slotting for spreading forwarders in time was introduced by Linda et al[26] and later used in Slotted p-persistence [25]. These approaches use a fixed number of slots and therefore cannot adapt to different node densities. Adaptive slotting based on workload and density has been proposed in some TDMA-based MAC protocols [27], [28], which are not compatible with 802.11p.